1. Field of the Invention
The invention relates to a nonlinear optical element used in the optical electronics field such as an optical signal processing element or an optical memory element which can control light, and a nonlinear optical structural body which can be used as material of the nonlinear optical element and can provide a nonlinear optical effect.
2. Discussion of the Related Art
In recent years, it has been desirable to develop optically controlled optical switching elements or optical memory elements which can control an optical output by a control light for realizing an optical information transmission apparatus or optical information processing apparatus which can process a large capacity and high speed information transmission and information processing.
An effect in which optical characteristics depend on the input optical intensity is referred to as a "nonlinear" effect. In general, when the input optical intensity is small, an optical response such as a reflected light spectrum or a transmitted light spectrum does not depend on the input optical intensity. These responses are referred to as linear responses because the amount of the induced polarization is proportional to the first order of the electric field in the medium. But, if the input optical intensity becomes large, for example, when two kinds of strong and weak lights are inputted, a phenomenon occurs in which a response of the weak light is varied by the strong light and a response of the strong light is varied by its own intensity. By utilizing such phenomena, an optical response such as a reflection or a transmission is controlled by its own light or other input lights.
At present, there are several types of nonlinear optical elements or nonlinear optical structural bodies studied for light-light control systems in which the light is controlled by the light. They are, for example, elements using the nonlinear optical effect occurred by the band filling effect using transition between bands in the GaAs quantum well and so on, elements using nonlinear motion of the free carriers in the semiconductor having narrow band gap such as InSb, or excitonic resonance type of optical material using a resonance of the exciton level in the bulk semiconductor, or exciton levels confined in the semiconductor quantum wells and microcrystallites. Optical switching elements and optical bi-stable elements also use the above elements.
As described above, optical switches driven by light-light control or optical memories are needed for an optical information processing system which processes a large capacity of information at high speed. In these elements, an interaction amount between lights becomes a big problem for controlling or processing light signals using the light. The light having no interaction in the vacuum gives an interaction between lights via polarization of its medium. The amount of interaction is decided by an amount of the intrinsic nonlinear polarization of the medium and depends on the intensity of the light.
FIG. 14 is a conceptual figure showing an operation of a conventional nonlinear optical element. In the figure, 10 is an optical medium which causes a nonlinear optical effect. 4 is input light provided into the optical medium 10. 5 is control light inputted into the optical medium 10. 6 is output light that the input light 4 passed through the optical medium 10. The intensity of the output light 6 is obtained by modulating the input light 4 and varying intensity of the control light 5 under the nonlinear optical effect of the optical medium 10. The reflection light is also modulated in the same way, but not shown in FIG. 14.
Generally, light-light interaction is very small in natural material. In order to carry out light-light control in the optical information processing, it is necessary to have very strong intensity of the control light. There has been no suitable example of a nonlinear optical element so far which can provide the light-light control using such a weak control light intensity which is sufficiently practical. One of the most important points for manufacturing the optical electronics element is how a large nonlinear optical effect can be caused by a weak control light intensity. This gives a solution whether the actual optical electronics element can be manufactured or not.
Therefore, in order to attain the actual light-light control, material or structure having a large nonlinear optical effect must be obtained by some methods. In order to obtain these materials or the structure, the nonlinear susceptibility is enhanced by artificially producing low dimensional material or very fine structures, in which electrons are quantum-mechanically confined and the oscillator strength is concentrated on a lowest excitation level. This is a major interest for quantum confinement effect of the electronic system.
But, in the material such as semiconductors having large interaction between atoms, a relation between the susceptibility and the actual response output is not so simple. Therefore, the amount of the nonlinear optical effect can not be fully evaluated by only evaluating the susceptibility of the material. The above limitation has not been well recognized so far, and also the theoretical method of evaluating the nonlinear optical susceptibility has not been correctly and well understood. Therefore, there is no established principle for producing material or structural bodies having a large nonlinear optical effect. Accordingly, no practical material and no structural body having such nonlinear optical body have been realized so far.
It is, therefore, a primary object of the present invention to provide a nonlinear optical element in which a sample size is selected so that the optical electric field, that is, the exciton polariton electric field, resonated with excitons, becomes maximum.
It is another object of the present invention to provide a nonlinear optical element having an enhanced nonlinear optical effect which operates with a remarkably weak input light or input control light.
It is another object of the present invention to provide a nonlinear optical element for use in a light-light switching element or an optical memory element of a light information system which is controlled by a control light.